Lower limb dieback (LLDB) has become an emerging problem within many almond orchards throughout California. Observations of orchards with LLDB occur independent of soil types, irrigation systems, and planting spacings, while typically affecting orchards that are in their 8th leaf or older. Symptoms are predominantly found on Padre and Butte, but are also found on many other varieties including Nonpareil, Fritz, Carmel, Wood Colony, and Mission.
The problem tends to be associated with smaller diameter branches in the lower canopy of orchards. Often enough, however, the problem extends to larger diameter branches, causing branch loss that can extend to 10 feet or more from the ground. Symptoms include wilted, yellow leaves that eventually fall from the tree. Bark removal will reveal a brown canker with little or no gumming that usually does not completely girdle the branch. The fungal canker can be observed on the top side of the affected branches, which is often found proximal to the yellowing leaves. Often, the canker is associated with a dead spur or small branch. The fungus appears to move up the branch to the point of attachment with the main scaffold, but does not appear to enter into the main scaffold. The first appearance of symptoms has been reported in mid-spring, with shoots continuing to collapse throughout the summer. Branch collapse is often noticeable about a week after a hot spell in which the evapotransipiration rates are very high.
Isolations made by Themis Michailides (UC Extension Plant Pathology Specialist) from almond trees in orchards affected by LLDB have identified two commonly isolated genera of fungi, Botryosphaeria andPhomopsis. Sampling of diseased limbs from 10 orchards in 2005 (Glenn, Madera, and Stanislaus Co.) and 18 orchards in 2006 (Butte, Colusa, Fresno, Glenn, and Kern Co.) identified Botryosphaeria spp. and Phomopsis spp. from 52% and 56% of the isolations made, respectively. Isolation of these pathogens was higher in late summer/fall than in spring/early summer. Interestingly enough, in the Sacramento Valley, fall isolations of these pathogens frequently occurred from both limbs with AND without symptoms. Corresponding pathogenicity tests of isolated fungi on thrifty and unthrifty trees indicated that the isolates of Botryosphaeria spp. and Phomopsis spp. can cause disease on almond trees. It was also found that the isolates of Botryosphaeria spp. tend to be more virulent than Phomopsis spp. There is much uncertainty of these fungi being the true cause of the disease, however, as they have been considered to be fungi that colonize weak tissues that are predisposed from some other stress.
Several fungicide trials by Roger Duncan (UCCE Stanislaus) have consisted of spring and fall applications of several fungicides. Fall (October – December) applications of copper hydroxide, liquid lime sulfur, Pristine©fungicide, Nutriphyte P© (0.5 gallons per acre), and Plant Shield©, a commercial formulation of Trichoderma harzianum(a biological control agent), did not lower the incidence of LLDB the following year. May applications of Captan 80 WDG©, Pristine©, and Agri-fos©, all applied with a bark penetrant, also failed to reduce LLDB symptoms. These trials indicate that chemical control for LLDB is either not possible or has not been identified.
Recent evidence suggests that orchard water management may play a primary role in the incidence of LLDB. Through the use of soil moisture monitoring systems and a pressure chamber, research by Bruce Lampinen (UC Extension Pomology Specialist) has demonstrated that orchards with LLDB frequently are over-watered in the early season (March – May, Northern Hemisphere, October – November, Southern Hemisphere). These studies have indicated that many soils are irrigated too soon in the growing season, saturating the deeper soil profile. Since roots need oxygen in order to grow and function, over-application of water prevents the diffusion of oxygen into the deeper soils. This leads to eventual root death, restricting the rooting profile. When temperatures increase in the spring and the water demand of the crop increases, the newly compromised root system struggles to provide enough water to meet the trees demands. At this point, trees begin to shed leaves that are producing fewer carbohydrates through photosynthesis. For the most part these leaves are on shaded spurs within the lower canopy. If the branch loses too many leaves, it is unable to remain active and is killed off by secondary pathogens (Botryosphaeria andPhomopsis) found within the environment.
Other orchard problems may also contribute to LLDB. Hull rot, scale infestations, and herbicide drift can damage the lower branches and kill spurs, providing an entrance for fungi. These problems occur frequently on younger trees (less than 8thleaf), before the onset of LLDB, which suggest that they may predispose the tree to LLDB. Therefore, it may be important for growers to implement orchard practices that will reduce damage to the lower canopy. Then again, any practice that reduces tree damage should be adopted to increase orchard longevity.
LLDB, although a problem, does not appear to have a negative impact on tree production. Many orchards within California have been struggling with this problem for over a decade, but still produce at a high production level. In observing this problem, it appears to be associated with high vigor, higher production orchards. Caution is urged when applying early season irrigations. Although preventing saturated soil conditions in the spring is not always possible, proper irrigation timing will help reduce the problem within orchards.